For well over a century, half the charm of Canova's marble nymph has gone to waste in the royal collection. The better half, to some minds.

"This is the first time anyone has been able to see her back, she's always been displayed shoved up against a wall," Christopher Lloyd, keeper of the Queen's pictures, said.

He looked down on the back, laid bare in the largest gallery in the Queen's dazzling new £20m picture gallery at Buckingham Palace. "Wonderful, isn't she? A sight no man could look on without feeling curiously moved."

For the opening exhibiton the curators have selected the gems of a collection regarded as one of the best in the world. It includes the latest acquisition, Lucian Freud's tiny grumpy portrait of the Queen, and the diamond diadem which he requested her to wear, both on display for the first time.

Two small treasure chambers are crammed with priceless objects, including a belt given to Queen Victoria, two of the huge flawless Cullinan diamonds, and a display case full of Fabergé toys including animals modelled on farm pets at Sandringham. The Fabergé display includes a cigarette case given by Camilla Parker Bowles' ancestor, Alice Keppel, to Queen Mary, which still holds a cigar half smoked by Mrs Keppel's lover Edward VII.

The largest object, Van Dyck's squash court sized equestrian portrait of Charles I, could only be got in through a tailormade door. The galleries also hold many objects from his famous collection.

The drawings gallery is an unbroken parade of master-pieces - just one wall has two Holbeins, a Raphael, a Michaelangelo and a Leonardo da Vinci. Alan Stewart, a fine arts graduate from Edinburgh, applied for the job of gallery attendant because of the drawings.

"To have the chance to stand beside a drawing by Leonardo all day - well that is just perfection."

The gallery is the most significant development at the palace in 150 years. It was created in space formerly occupied by kitchens and a police guard room, one of the innumerable extensions to Buckingham palace.

The gallery will open to the public on Wednesday, after Tuesday's formal opening by the Queen - who may find one feature reassuringly grand yet curiously domestic.

Most of architect John Simpson's decoration was inspired by John Nash's 1831 picture gallery. However he had no model for the elaborate balustrade of the stone stair which sweeps up from the entrance hall. This features patinated bronze columns and scrolls, linked by gilt ironwork swags which curiously resemble... The architect giggled: "Ah yes. The gold dressing gown cords. Rather my own idea."